內容有float時外框適應高度

當內容有float時 外框就不會自動感知高度了
此時請服用clearfix
.clearfix:after {
    content: ".";
    display: block;
    height: 0;
    clear: both;
    visibility: hidden;
}

.clearfix {display: inline-block;}

/* Hides from IE-mac \*/
* html .clearfix {height: 1%;}
.clearfix {display: block;}
/* End hide from IE-mac */

在外框的class加上clearfix唷!